This invention is directed to gas furnace controllers, and more particularly, to a gas furnace controller which allows a single stage thermostat to control a two stage furnace. This invention is also applicable to the controllers of the heating and cooling elements of an HVAC system.
In heating applications, a two stage thermostat typically controls a two stage furnace by sending a first call for low heat on a first control line (W1) and a second call for high heat on a second control line (W2). However, under some circumstances, a single stage thermostat is used to control the two stage furnace. Under such circumstances no call for high heat is made on the second control line W2 because the single stage thermostat does not include the capability to issue such a call. Exemplary circumstances include retrofit or upgrade applications where an existing single stage thermostat is retained when the original single stage furnace is replaced with a two stage furnace. However, the two stage furnace cannot operate its second stage unless the furnace recognizes that the furnace is under the control of a single stage thermostat, and unless the furnace recognizes when to operate the second furnace stage.